1. Field of the Invention
The present invention is generally related to turbidity sensors and, more particularly, to a turbidity sensor that is provided with a means for controlling the intensity of a light source for the purpose of compensating for variability of light emitting characteristics of the light source.
2. Description of the Prior Art
Turbidity sensors are used in many different types of applications. Some turbidity sensors are used in association with machines for washing articles, such as dishwashers and washing machines. Most turbidity sensors measure the effect on a light beam by particulate matter suspended within a fluid. Some turbidity sensors use only a transmitted light signal while others use both scattered and transmitted light signals.
U.S. Pat. No. 4,257,708, which issued to Fukuda on Mar. 24, 1981, discloses an apparatus for measuring the degree of rinsing in a washing machine. It is provided with a source of light, a first phototransistor disposed to receive light emitted by the light source for producing a reference signal a second phototransistor disposed to receive the light from the light source for producing a measuring signal corresponding to the amount of light received and a calculating circuit for arithmetically operating the reference signal and the measuring signal for producing an output signal corresponding to the relative values of the reference signal and the measuring signal. A first optical path between the light source and the first phototransistor and a second optical path between the light source and the second phototransistor are both disposed in rinsing water and the length of the first optical path is set to be longer than a length of the second optical path.
U.S. Pat. No. 5,291,626, which issued to Molnar et al on Mar. 8, 1994, describes a machine for cleansing articles, such as a dishwasher, that incorporates a device for measuring the turbidity of an at least partially transparent liquid. The device includes a sensor for detecting scattered electromagnetic radiation, regardless of polarization, and a sensor for detecting transmitted electromagnetic radiation, regardless of polarization.
U.S. Pat. No. 5,172,572, which issued to Ono on Dec. 22, 1992, discloses an automatic washing apparatus for washing dirty items in a washing tank to which washing liquid is supplied. The automatic washing apparatus comprises a light emitting element for emitting light to the washing liquid which is passed through the washing tank. It also comprises a first light receiving element for receiving a linear light beam which travels through the washing liquid along the optical axes of the light emitting element, and a second light receiving element for receiving scattered light which travels through the washing liquid in directions deviated from the optical axis of the light emitting element, wherein washing conditions are controlled in accordance with the quantity of light received by the first light receiving element and the quantity of light received by the second light receiving element.
One problem that can be encountered in the operation of a turbidity sensor is the variability of the light intensity provided by a light source. If a light emitting diode is used as the light source, several factors can disadvantageously affect the intensity of light provided by the diode. For example, changes in temperature will affect the light emitted from a light emitting diode for a particular current flowing through the diode. In addition, the intensity of light from a light emitting diode decreases as the diode ages. In addition to these problems, the uniformity of the light emitting characteristics of diodes vary greatly and no two diodes from a particular batch can be assumed to be identical in their light emitting characteristic. Since most turbidity sensors operate as a function of the intensity of light received by one or more light sensitive components, it is important that the intensity of light emitted by the light source be either constant or known. It would therefore be significantly beneficial if a means were provided to assure the constancy of the light intensity from a light source in a turbidity sensor.